Certain gas turbines for automotive plants, operating according to a conventional working process, include a gas producing portion comprising a compressor and at least one turbine rotor driving the same, as well as a turbine part, comprising at least one power turbine rotor and at least one auxiliary turbine rotor. The three types of rotors are interconnected by transmission means permitting transfer of power from the auxiliary rotor to the compressor rotor and/or to the power turbine.
With the conventional working process it is very difficult, during low loads upon the plant, to obtain a satisfactory efficiency and a low fuel consumption during idle running. The drawbacks may be reduced by the use of adjustable guide vanes (variable geometry) at the turbine part, as well as at the compressor part, and by using an automatically variable transmission with a divisible torque flow and/or a direct reduction gear. That will, however, imply considerable cooling problems, and also high production costs, in relation to the low power required for small cars. The present invention provides a solution of above mentioned difficulties, and proposes a novel, combined working process, where one of the rotors has been formed and introduced in a basically novel manner, whereby it will operate as a turbine (during high load), or as a turbo-fan (during part load), and whereby further one or more stator members may be simplified, or omitted.
When a conventional gas turbine plant, having stationary guide vanes, operates at part load the flow of gas through the plant will be reduced by the speed and the pressure being lowered. The maximum turbine temperature (Tmax), and thus also the thermal efficiency, will then be reduced, and it is difficult to maintain an acceptable part-load economy. The highest, theoretical thermal efficiency (.eta..sub.tt) and the average thermal efficiency (.eta..sub.t), which is proportional to the highest efficiency, are dependent upon Tmax; according to .eta..sub.tt =(Tmax-To) /Tmax and it is evident, that it, also during part-load operation, is desirable to maintain the temperature as high as possible. In order to maintain a high Tmax, and a corresponding high .eta..sub.tt. it is necessary that the specific load upon the compressor turbine is maintined at a high level, or preferably is raised, when the pressure drop across said turbine, and the temperature, are reduced and/or the volume of air (gas) is reduced. The Tmax should preferably be raised during part-load operation in order to compensate other reducing actions during part-load. This may be acceptable with respect to resulting strains, as the mechanical stresses upon the rotor components will be reduced due to the lowered speed. A balancing will, however, have to be made with respect to heat losses (insulation) and oxidation (surface treatment ceramics).